In　recent　years,　the　phenomenon　of　total　nitrogen　loss　which　is　found　in　single　aerobic　or　oxygen-limited　wastewater　treatment　systems　has　attracted　researchers＇　concern.　Microbiological　theory　and　related　studies　of　nitrogen　loss　were　reviewed　in　this　paper.　It　involves　the　different　groups　of　bacteria　alone　or　in　cooperation,　including　nitrogen　removal　by　autotrophic　nitrifying　bacteria　and　aerobic　denitrifying　bacteria,　the　inter-synthesis　between　nitrifying　bacteria　and　aerobic　denitrifying　bacteria;　and　the　cooperation　of　nitrifying　bacteria　and　anaerobic　ammonia　oxidation　(Anammox)　bacteria.　The　new　biological　nitrogen　removal　mechanisms,　which　have　incomparable　advantages　to　the　traditional　nitrification-denitrification　nitrogen　removal　technique,　have　important　significance　to　enhance　nitrogen　removal　in　wastewater　treatment.　Fig　8,　Ref　53.